1. Field of Invention
The invention relates to a mixture solution for preparing a conductive polymer to produce a solid-state electrolytic capacitor. The conducting polymer is formed from the mixture of monomer and oxidant solution. Particularly, the oxidant solution is high concentration, and is also added a five or six-member ring compound with a functional group
which acts as a retardant for the polymerization.
2. Related Art
For a long time, the development of electrolytic capacitors has been focusing on improving the conductivity of electrolyte so as to reduce the equivalent series resistance (ESR) and impedance and improve the reliability of capacitors. Because conductive polymer has higher conductivity than liquid-state electrolyte or solid-state organic semiconductor complex salt (such as TCNQ complex salt) and it becomes insulator in higher temperature, it is perfectly used as the electrolyte of electrolytic capacitors.
In U.S. Pat. No. 4,803,596, Gerhard Hellwig, Stegen et al. first disclosed that conductive polymer would be used as the electrolyte of an electrolytic capacitor. After the positive foil of an electrolytic capacitor is dropped with the monomer and oxidant solution, respectively, the monomer polymerizes by the oxidant under proper condition. By this method, however, the conductive polymer monomer is not fully mixed with the oxidant, so the reaction is not uniform.
Therefore the method for producing conductive polymer electrolytes of solid-state electrolytic capacitors is improved by mixing conductive polymer monomer with dopant, oxidant and solvent, and then making the capacitor element impregnated with such the mixture solution. Finally the monomer absorbed in the element converts into conductive polymer. If the reactivity of the oxidant in the mixture is not inhibited, however, the conductive polymer monomer polymerizes as soon as it mixes with the oxidant. As a result, the mixture solution is unable to permeate into the pores of the capacitor element.
Friedrich Jonas et al. disclosed in U.S. Pat. No. 4,910,645 a series of specific polythiophenes can apply to the electrolyte of solid state electrolytic capacitors. The method is simply to dip the capacitor element in the mixture of thiophene monomer and oxidant solution without polymerization retardant and then thiophene monomer polymerize at high temperature. Unfortunately, the stability of the mixture of the thiophene monomer and oxidant solution at room temperature decreases with the high concentration both of the monomer and oxidant. Therefore the proposed method uses a large amount of solvent to dilute the monomer and oxidant. As a consequence, when a capacitor element is impregnated with the mixture solution, it only obtains a little conductive polymer, and the rest is the remaining reagent and a large amount of solvent. Multiple steps of impregnation and polymerization processes are required by this method in order to produce enough conductive polymer to fill the space in a capacitor element.
Phipip M. Lessner et al. disclosed in U.S. Pat. No. 6,056,899 a process that uses a kind of of cyclic ethers (such as tetrahydrofuran) to mix with an oxidant of ferric ion to produce a complex, reducing the oxidation ability of the oxidant so that the mixture solution of the conductive polymer monomers and oxidant is kept stable for a long period of time. After the capacitor element is impregnated with the mixture solution, the cyclic ether is evaporated at a higher temperature to make the oxidant induce a polymerization of conductive polymer. Because the cyclic ethers disclosed in the invention (such as tetrahydrofuran) have little ability as a polymerization retarder to stabilize the mixture solution of monomer and oxidant, a large amount of such cyclic ether is consumed in order for it to stabilize the mixture solution and, at the same time, dilutes the mixture solution of conductive polymer monomer and oxidant. As a result, a capacitor element still needs to be impregnated and polymerized many times to acquire a sufficient amount. Thus, this patent still has the shorcomings of fabricating solid-state electrolytic capacitors by a complicated process.